Most modern vehicles have power steering in which the force exerted by the operator on the steering wheel is assisted by hydraulic pressure from an electric or engine-driven pump. The force applied to the steering wheel is multiplied by the mechanical advantage of a steering gear. In many vehicles, the steering gear is a rack and pinion, while in others it is a recirculating ball type.
Electric power steering is commonly used in the hybrid vehicles to improve fuel economy and has started to replace hydraulic power steering in some vehicles. One way this is accomplished is through the reduction or elimination of losses inherent in traditional steering systems. Therefore, electric power steering typically requires power only on demand. Commonly, in such systems an electronic controller is configured to require significantly less power under a small or no steering input condition. This dramatic decrease from conventional steering assist is the basis of the power and fuel savings. Electric power steering has several additional advantages. The steering feel provided to the operator has greater flexibility and adaptability. Overall system mass savings may also be achieved. Electric power steering is powerplant independent, which means it can operate during an all-electric mode on a vehicle.
Furthermore, polyphase permanent magnet (PM) brushless motors excited with a sinusoidal field provide lower torque ripple, noise, and vibration when compared with those excited with a trapezoidal field. Theoretically, if a motor controller produces polyphase sinusoidal currents with the same frequency and phase as that of the sinusoidal back electromotive force (EMF), the torque output of the motor will be a constant, and zero torque ripple will be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from pure sinusoidal back EMF and current waveforms. Such deviations usually result in parasitic torque ripple components at various frequencies and magnitudes. Various methods of torque control can influence the magnitude and characteristics of this torque ripple.
One method of torque control for a permanent magnet motor with a sinusoidal, or trapezoidal back EMF is accomplished by directly controlling the motor phase currents. This control method is known as current mode control. The phase currents are actively measured from the motor phases and compared to a desired profile. The voltage across the motor phases is controlled to minimize the error between the desired and measured phase current. However, current mode control requires a more complex controller for digital implementation and processing. The controller would also require multiple current sensors and A/D channels to digitize the feedback from current sensors, which would be placed on the motor phases for phase current measurements.
Another method of torque control is termed voltage mode control. In voltage mode control, the motor phase voltages are controlled in such a manner as to maintain the motor flux sinusoidal and motor back emf rather than current feedback is employed. Voltage mode control also typically provides for increased precision in control of the motor, while minimizing torque ripple. One application for an electric machine using voltage mode control is the electric power steering system (EPS) because of its fuel economy and ease-of-control advantages compared with the traditional hydraulic power steering.
In voltage mode control the amplitude and phase angle of phase current vector is calculated based on the motor back emf, position and motor parameters (e.g., resistance, inductance and back emf constant). A sinusoidal instantaneous line voltage based on the calculated phase and amplitude vector of phase voltage is applied across the motor phases. An instantaneous value of voltage is realized across the phases by applying a pulse width modulated (PWM) voltage the average of which is equal to the desired instantaneous voltage applied at that position of the motor.
There are different methods of profiling the phase voltages in order a achieve a sinusoidal line to line voltage and therefore the phase current in a wye-connected motor. A conventional approach is to apply sinusoidal voltages at the phase terminals. In this method the reference for the applied voltage is at half the dc bus voltage (Vdc/2). Another approach is the phase to grounding, phase voltage method, which increases the voltage resolution and reduces switching losses. In this method, the phase voltage is referenced to the power supply ground (instead of Vdc/2 as in conventional way). This ground reference is achieved by applied a zero voltage for 120 electrical degrees at each phase terminal during one electric cycle.
EPS control systems employing voltage mode control algorithms, generally use the amplitude and phase angle of the voltage for torque control. In order to produce the accurate torque from the motor it is important to apply and control both amplitude of the voltage and its phase angle as accurately as possible. Errors in the phase angle may produce inaccurate torque. An error high enough to cause the angle between back emf and current of more than 90 degrees, may cause a reversal of torque and thus the motor direction. Therefore, in a voltage control system, it may be desirable to monitor the motor voltage phase angle and determine if it appears to be reasonable.
A method and system for phase angle diagnostics in a sinusoidally excited PM electric machine, including: a controller that executes a process to facilitate the method including receiving a position value indicative of the rotational position from a position sensor configured to measure a rotor position of said electric machine and transmit a position signal; receiving a phase advance value indicative of a commanded phase advance angle; determining an expected phase voltage and expected status thereof; observing a selected phase voltage signal and determining an actual status corresponding thereto; and comparing the expected status and the actual status to ascertain the phase angle reasonableness.
A storage medium encoded with a machine-readable computer program code for phase angle diagnostics in a sinusoidally excited PM electric machine, the storage medium including instructions for causing controller to implement the disclosed method.
A computer data signal embodied in a carrier wave for phase angle diagnostics in a sinusoidally excited PM electric machine, the data signal comprising code configured to cause a controller to implement the disclosed method.